In the production of metal frames used to separate the two sheets of glass used in thermally insulating glass sold under the name of Thermopane it is necessary to attach the straight hollow mitered metal bars together very securely. It is almost invariably necessary to make each thermal pane to order, that is, to a specific size required by the customer since it is impossible to cut down a finished sheet. The bars must be cut by a skilled worker and mitered accurately, which is a relatively simple operation. Then it is conventional to secure the corners together with small L-shaped reinforcements that fit inside the hollow bars. Finally each corner is soldered or brazed. This latter operation is usually carried out by pulling the assembled frame around on a large flat table and maneuvering each corner into a treatment station where it is soldered or brazed. This latter operation is difficult and takes up an inordinate amount of room.